Tunable particle-free region via acoustic force balance around an oscillating microbubble

Acoustically oscillating microbubbles have demonstrated considerable potential for non-contact particle manipulation in microfluidic systems. While their vortical microstreaming is widely used for particle trapping, the physics mechanisms underlying stable, long-range repulsion and its tunability remains insufficiently explored. This study systematically investigates a tunable particle-free region (PFR) generated by a single sessile microbubble pinned at the rim of a 40μm cylindrical microhole and driven by a 165 kHz acoustic field, using a combined experimental and numerical approach. Through high-speed imaging, laser Doppler vibrometry (LDV), and particle image velocimetry, we have revealed that the PFR is formed by the balance between the short range primary acoustic radiation force and the inward drag associated with the outer recirculating branch of steady microstreaming. The position of this equilibrium boundary can be precisely modulated by the LDV measured microbubble radial oscillation amplitude, ΔR. For 2 μm polystyrene particles, the PFR area increases systematically with ΔR, reaching a maximum of 1.7×10⁵μm² at ΔR = 1569.7 nm, beyond which nonlinear oscillations lead to interface instability and ultimately cause microbubble rupture. Complementary thermoviscous acoustics simulations quantitatively reproduce the streaming structure and its dependence on the microbubble oscillation amplitude, providing direct support for the proposed force balance mechanism. A scaling analysis further reveals that particle inertia is the dominant constraint for larger particles, resulting in the formation of smaller PFRs. By elucidating the coupled physics of acoustic forces, microstreaming, and particle inertia, this research establishes a theoretical and experimental framework for the predictable generation of tunable, purified regions, with significant implications for applications in biosensing and single-cell analysis.